The present invention relates to a screw extruder for mixing, homogenizing, and degassing highly viscous rubber mixtures and similarly acting thermoplastics; the extruder includes a housing having a cylindrical bore and a rotatable, permanently mounted screw conveyor which has an entry segment, a mixing and homogenizing segment, and a degassing segment; the screw chamber of the screw conveyor is connected with the atmosphere or a vacuum source via a line, channel, or the like in the housing; also provided are pins, studs or other obstacle means which are distributed over the periphery of the housing and are adapted to extend from the housing radially inwardly into the space, i.e. screw chamber, between the screw conveyor and the housing.
When rubber mixtures are extruded into profiled articles, hoses, cords, etc., as well as during cable covering or sheathing with subsequent, generally pressureless, vulcanization, air or/and moisture present in the mixture can lead to an undesired porosity of the vulcanized goods.
Rubber mixtures or thermoplastic compositions are degassed in order to eliminate these deficiencies in quality. This degassing occurs in so-called degassification regions or vacuum zones of the respectively employed extruder. The mixture is dosed subsequent to a retaining or damming means into an enlarged volume zone in which the mixture can expand and new surfaces formed can be degassed.
According to U.S. Pat. No. 1,283,947 Steinle issued Nov. 5, 1948, it is known to provide the screw conveyor with a retaining or obstruction ring, or split ring, in the degassing region. An evacuation channel or passage is arranged in the housing directly after the ring. Furthermore, pins are secured to the housing in this region. The rubber mixture is first cut or sheared by the narrow ring gap, thereby receiving a film-like form which is separated into individual parts by the pins. The gases and moist particles are suctioned off in this phase. The evacuation channel or passage is provided for the entire degassing zone. A high vacuum must under certain circumstances be applied in order to be able to effectively degassify. Even then, not all trapped gas particles or moisture particles can be removed through this single channel.
According to German Offenlegungsschrift No. 21 60 062, it is known to provide the screw conveyor with a retaining or obstruction ring in the degassing region, and to provide the housing with an axially displaceable sleeve in order to provide regulation of the rate of flow of the conveyed mass of synthetic material prior to degassing. Additional aids for dividing the material can be provided in the form of longitudinal ribs on the screw conveyor, and/or pins secured to the housing. Degassing is effected with the aid of a single degassing channel or passage arranged in the housing. Such a degassing channel is generally tunnel-shaped in the screw conveyor housing wall.
A further extruder, disclosed by Swiss Pat. No. 510 512, has such a channel or tunnel for degassing. This extruder is further provided with auxiliary means, for releasing the gases, in the form of oscillation-generating ultrasonic devices. Roller bodies or pistons can also be employed.
All known screw extruders have available a degassing channel through which the rubber mixture or synthetic material mass is degassed by virtue of the applied vacuum. An effective degassing requires a homogeneous preparation in the mixing and homogenizing zone, and a mass conveyed into the degassing region as constant as possible with regard to the volume, temperature and viscosity. Additionally, the degassing or evacuation of the material, which exists in an expanded state in the degassing region, is to occur if possible at many locations simultaneously. This, however, is not possible with the known degassing screw extruders.
It is an object of the present invention effectively and advantageously to degassify a mechanically and thermally satisfactorily uniform mass during the processing of highly viscous rubber mixtures and similarly acting thermoplastics with a screw extruder of the aforementioned general type while avoiding significant shear flow due to splitting up the conveyed mass or material into numerous rearranged, surface-enlarged partial material flows, and to thereby avoid the disadvantages of the heretofore known degassifying extruders.